Division of Radiooncology, Deutsches Krebsforschungszentrum, 69120 Heidelberg, Germany
For the visualization of specific molecular details in the electron microscope it is necessary to create what is known as defined image contrast: regions of varying electron opacity that allow characteristic differences to be detected and information about the structure to be discerned. In order to distinguish cellular antigens by immunohistology, the employed antibodies (as well as other molecular probes) must be “labeled” so that the resulting complex becomes visible. Substances which lead to significant deflection of the electrons (or to appropriate signals) in the electron microscope and which remain stable under the various steps of tissue preparation are suitable for labeling purposes. Typical marker substances may be classified as (1) primary electron dense molecules such as ferritin; (2) particulate substances detectable by their characteristic shape and size such as plant viruses and colloidal gold; (3) secondary electron dense labels such as enzymes due to specific substrate conversion.